Saves screenshot of the specified locator to ouput folder (set in codecept.conf.ts or codecept.conf.js).
Filename is relative to output folder.

```js
I.saveElementScreenshot(`#submit`,'debug.png');
```

@param {CodeceptJS.LocatorOrString} locator element located by CSS|XPath|strict locator.
@param {string} fileName file name to save.
@returns {void} automatically synchronized promise through #recorder
